Recent years traditional fluorescent tubes have been modernized in that the outer features of the tube and the electric connection parts have been kept but the light engine has been replaced with modern technology of one or more solid state light emitting elements, such as LEDs (Light Emitting Diodes), and OLEDs (Organic Light Emitting Diodes), etc. One example thereof is EnduraLED T8 manufactured by Philips. Typically, several solid state light emitting elements are mounted in a line on a carrier, which is introduced into a glass tube, and the inside of the glass tube is provided with a light diffuser, which diffuses the spot shaped light from the solid state light emitting elements into a homogeneous light output. Present light diffusers obtain the diffusing effect by a combination of reflection and scattering transmission of the light. However, in order to obtain a good uniformity of light distribution the solid state light emitting elements have to be densely mounted or the light diffuser has to be reflective to a high extent. A high reflectivity causes a low optical efficiency. Densely mounted solid state light emitting elements cause a high cost.